Wednesday, August 05, 2009


The story of the beach between the mouth of the Elwha River and Port Angeles is well known – or at least often told. Dams, riprap, and a spit starved of sediment. Simple stories are easy to tell. There’s a longer, more complicated story, and one that I won’t try to tell here, in part because there are others working hard on figuring it out, but here’s snippet.

Years ago, I noticed in older aerial photos that the character of the beach between the delta and Dry Creek
seemed to change significantly on a decadal scale. Photos from the 1970s show a wide gravel berm and a heavily vegetated bluff, while photos from the 1990s showed a narrow berm and a bare, eroding bluff. By the beginning of this decade, the beach was widening again in locations and the bluff seemed to be responding by growing trees. When I walked this stretch in 2005, the beach showed evidence of the building seaward in sections, but for the most part the berm farther east was relatively narrow (beach level photos).

Last week, I flew over this stretch of shoreline and took this aerial of the same stretch. Wow! Whatever the mechanics – longshore drift, onshore transport – that’s a lot of gravel!

In the spirit of multiple working hypotheses…. The Elwha probably releases sediment to the beach in pulses, maybe associated with avulsion of the spits that regluarly form across its mouth, and one would expect these to travel downdrift in a somewhat coherent fashion. Or alternatively, there are oceanographic or meteorological factors that lead to long-term cycles of accretion and erosion. Or maybe, longshore gravel transport in highly oblique wave environments is influenced by nonlinearities that give rise to emergent landforms (I’ve probably butchered the jargon). Or maybe all three.

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